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    <title>Document</title>
    <style>
        * {
            margin: 0;
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        div {
            height: 500px;
            background-color: pink;
        }
    </style>
</head>

<body>
    <div>
        <span></span>
    </div>
    <script>
        var div = document.querySelector('div');
        console.log(div.offsetWidth);
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</body>

</html>